Many force-measuring devices, in particular gravimetric measuring instruments such as for example balances, thermogravimetric instruments, instruments for the gravimetric determination of moisture content, weighing modules for tank installations and reactor containers, weighing modules and multiple weighing devices in filling and packaging systems, as well as measuring devices for torque and acceleration are protected by effective measures against destruction in view of their sometimes very aggressive work environment. These protective measures consist of housings which are adapted to the work environment and which have to meet specific regulatory requirements in regard to the penetration of dust, moisture and the like, of the kind that are defined for different protection classes for example in the norm document EN60529, Ingress Protection Ratings.
High-capacity force-measuring cells for weighing modules, so-called tank- or reactor-container weighing modules are for example enclosed in stainless steel housings that are welded gas-tight. The force-measuring cells encased in such housings work problem-free as long as the environmental factors which influence the weighing signal are kept away from the force-measuring cell by the housing. Also, in most cases, the force measuring cells are not instantaneously destroyed in the case of housing leaks, but the destruction occurs more as a gradual process which is often discovered only at a late stage. If the measuring device is built into an industrial installation with a high degree of automation, a defect in the measuring device can often lead to a long downtime of the system or to defective products.
Depending on the conditions of the ambient environment, it is not absolutely necessary for the force-measuring devices to be hermetically encapsulated. Housings of a simpler and more cost-effective kind with contact-free passage openings as disclosed for example in U.S. Pat. No. 6,923,449 B2 can also be used in an industrial environment. Conventional balance housings, too, can meet their purpose adequately under suitable ambient conditions. Still, with an error in manipulation it is possible that for example a liquid penetrates into the interior of the housing and increases the relative humidity of the housing interior to such a degree that parts of the force-measuring cell or the electronic components of the signal-processing circuitry become corroded.
Force-measuring devices are often transported over long distances and kept in storage before they are put into operation. With exposure to unsuitable environments during transportation and storage, it is possible that condensation occurs inside the housing, which can severely compromise the measurement performance.
Depending on the housing enclosure, an inspection of the force-measuring cell is very expensive or even impossible. A periodic checking of the force-measuring devices used in systems installations is complicated and expensive.
A force-measuring cell which is disclosed in U.S. Pat. No. 7,051,603 B1 is in addition equipped with a temperature sensor. The analog signals of the force-measuring cell are converted by means of a first converter circuit, and the analog signals delivered by the temperature sensor are converted by a second converter circuit into bi-level pulsewidth-modulated signals. These signals are transferred by way of connecting leads to a processor module, where they are further processed by means of compensation data which are recalled from a memory storage module. The processing of the force-measuring cell signals with the temperature sensor signals serves to correct the temperature-related drift of the weighing cell. While this kind of treatment of the weighing signal can adequately compensate for the effects of the ambient environment on the weighing result, it cannot provide a determination of the actual condition of the force-measuring cell.
The present objective, therefore, is to provide a method for the monitoring and/or determination of the condition of a force-measuring cell arranged inside a housing without the need to open the housing in order to determine the condition of the force-measuring cell.